Cross-resistance to alpha-cypermethrin after xanthotoxin ingestion in Helicoverpa zea (Lepidoptera : Noctuidae)

Cytochrome P450 monooxygenases (P450) are membrane-bound hemoproteins that play important roles in conferring protection against both naturally occurr ing phytochemicals and synthetic organic insecticides. Despite the potentia l for common modes of detoxification, cross-resistance between phytochemica ls and synthetic organic insecticides has rarely been documented. In this s tudy, we examined the responses of a susceptible strain of corn earworm, He licoverpa zea (Boddie), a polyphagous noctuid, to exposure by an allelochem ical infrequently encountered in its host plants and by an insecticide wide ly used for control purposes. Within a single generation, survivors of xant hotoxin exposure displayed higher levels of tolerance to alpha-cypermethrin than did unexposed control larvae. The F-1 offspring of xanthotoxin-expose d survivors also displayed higher alpha-cypermethrin tolerance than did off spring of unexposed control larvae, suggesting that increased alpha-cyperme thrin tolerance after xanthotoxin exposure represents, at least in part, he ritable resistance. Administration of piperonyl butoxide, a P450 synergist, demonstrated that resistance to both xanthotoxin and alpha-cypermethrin is P450-mediated. alpha-Cypermethrin-exposed survivors, however, failed to sh ow superior growth on xanthotoxin diets. Assays with control larvae, larvae induced, by both xanthotoxin and alpha-cypermethrin, and survivors of LD50 doses of both compounds indicated that H. tea midgut P450s are capable of metabolizing both xanthotoxin and alpha-cypermethrin. Metabolism of each co mpound is significantly inhibited by the presence of the other compound, su ggesting that at least one form of P450 in H. sea midguts degrades both com pounds and may constitute the biochemical basis for possible cross-resistan ce. Compared with control larvae, xanthotoxin- and alpha-cypermethrin-induc ed larvae displayed 2- to 4-fold higher P450-mediated metabolism of both co mpounds. However, xanthotoxin- and alpha-cypermethrin-exposed survivors exh ibited much higher ( 2.5- to 11-fold) metabolism of both compounds than did the induced larvae. The metabolism results, like the bioassay results, are consistent with the interpretation that increased alpha-cypermethrin toler ance after xanthotoxin exposure is attributable mainly to heritable resista nce.